Ocean island basalt

The term Ocean Island Basalt (also known as OIB) is used to describe volcanic rock, usually basaltic in composition, erupted in the oceans away from tectonic plate boundaries. It is worth noting that while the name suggests that ocean island basalts are basaltic in composition, a wide range of compositions can be found at some ocean island basalt locations. For example, Iceland is home to ocean island basalts that range in composition from tholeiitic basalt to rhyolite, and the intraplate volcano Fernando de Noronha also hosts ocean island basalt with diverse compositions, such as phonolites and trachytes. Unlike mid-ocean ridge basalts (MORBs), which erupt at spreading centers (divergent plate boundaries), and arc lavas, which erupt at subduction zones (convergent plate boundaries), ocean island basalts are the result of intraplate volcanism. However, some ocean island basalt locations coincide with plate boundaries like Iceland, which sits on top of a mid-ocean ridge, and Samoa, which is located near a subduction zone. The term Ocean Island Basalt (also known as OIB) is used to describe volcanic rock, usually basaltic in composition, erupted in the oceans away from tectonic plate boundaries. It is worth noting that while the name suggests that ocean island basalts are basaltic in composition, a wide range of compositions can be found at some ocean island basalt locations. For example, Iceland is home to ocean island basalts that range in composition from tholeiitic basalt to rhyolite, and the intraplate volcano Fernando de Noronha also hosts ocean island basalt with diverse compositions, such as phonolites and trachytes. Unlike mid-ocean ridge basalts (MORBs), which erupt at spreading centers (divergent plate boundaries), and arc lavas, which erupt at subduction zones (convergent plate boundaries), ocean island basalts are the result of intraplate volcanism. However, some ocean island basalt locations coincide with plate boundaries like Iceland, which sits on top of a mid-ocean ridge, and Samoa, which is located near a subduction zone. In the ocean basins, ocean island basalts form seamounts, and in some cases, enough material is erupted that the rock protrudes from the ocean and forms an island, like at Hawaii, Samoa, and Iceland. Over time, however, thermal subsidence and mass loss via subaerial erosion causes islands to become completely submarine seamounts or guyots. Many ocean island basalts erupt at volcanic hotspots, which are thought to be the surface expressions of melting of thermally buoyant, rising conduits of hot rock in the mantle, called mantle plumes. Mantle plume conduits may drift slowly, but Earth’s tectonic plates drift more rapidly relative to mantle plumes. As a result, the relative motion of Earth’s tectonic plates over mantle plumes produces age-progressive chains of volcanic islands and seamounts with the youngest, active volcanoes located above the axis of the mantle plume while older, inactive volcanoes are located progressively farther away from the plume conduit (see Figure 1). Hotspot chains can record tens of millions of years of continuous volcanic history; for example, the oldest seamounts in the Hawaiian hotspot chain are over 80 million years old.